1. Field of the Invention
This invention relates to a pulse width modulation control unit of an inverter, and more particularly, to an improvement in the precise waveform control effected by increasing the carrier frequency.
2. Description of the Prior Art
As a high-speed switching device, elements such as MOSFETS (metal oxide film gate field effect transistors) have been developed in recent years. The pulse width modulation control unit of an inverter in which MOSFETS were introduced is already known from U.S. Pat. No. 4,691,269, for example. This control unit is capable of effecting precise waveform control and therefore a reduction in electromagnetic noise and an improvement in motor efficiency can be expected from it.
In the above cited U.S. patent, as disclosed in its specification from line 48, column 3 to line 62, column 4, the control of the pulse width modulation by a high carrier frequency is made possible by providing special hardware composed of analog control circuits. The control of the pulse width modulation by a high carrier frequency (20 KHz, for example) is also possible using a high-speed arithmetic operator, such as special hardware composed of digital circuits, a DSP (digital signal processor), or the like.
Also, control of the pulse width modulation of an inverter using a microcomputer is known from U.S. Pat. No. 4,622,628.
However, the use of a high-speed arithmetic operator, such as hardware composed of analog control circuits or digital circuits involves such drawbacks as intricacy of circuits, complexity of various regulations, higher costs, etc.
It is therefore conceivable to adopt a one-chip microcomputer which is low in cost and simple in circuit composition. This idea, however, as stated in "Power Electronics and AC Drivers" (written by B. K. Bose, published by Prentice-Hall Inc.), involves a long operating time of a microcomputer (about 200 .mu.S) for a series of processes required for the generation of PWM control patterns and this operating time corresponds to a carrier frequency of about 5 KHz at a maximum. Therefore, the pulse width modulation control by a high carrier frequency (more than 20 KHz) is difficult.